Ground flares and incinerators are being used more frequently as they are typically more environmentally efficient. Regulations are being tightened with emissions resulting from flaring, venting of tank vapors and venting of BTEX emissions (benzene, toluene, ethylbenzene and xylene) from the glycol dehydrators on natural gas wells.
Waste gases from the wellsite and gas treatment facilities are incinerated in ground flares at high temperature to ensure that complete combustion takes place. The majority of the combustion takes place within the burning chamber and the stack and, unlike open flares, there is usually no visible flame outside the stack. A ground flare burns its fuel in a chamber in the flare stack and, as a result, combustion is more controlled. Oil and gas industry studies have shown that combustion efficiency drops significantly when combustion takes place outside the stack and worsens as outside wind increases. U.S. Pat. No. 4,652,233 to Hamazaki utilizes a conventional burner extending into the combustion chamber and emphasizes the wind proofing of the stack to ensure efficient combustion.
As is the case when waste gases, having fluctuating quality, are burned, the burners sometimes need to be serviced or changed out to a style or size appropriate to the quality and quantity of gas presently being combusted. With the conventional burner systems, the burners cannot be changed while waste gas continues to be burned; instead the facility must be shut in or re-routed to other equipment during servicing.
Usually ground flares do not use forced air, relying on induced draft to supply combustion gases. The burners typically utilize a gas header with upwardly extending nozzles for atomization of the waste gas upwardly into the combustion chamber. While it is known to remove one of multiple forced air burners from furnaces without interrupting operation, it is not known to remove a gas header bearing nozzles from a ground flare stack. The vertically oriented nozzles significantly encumber the horizontal in-stack gas header and complicate its removal therefrom.
The apparatus disclosed by Hamazaki is complicated, as is the apparatus of other ground flares known to the applicant and they do not disclose means for dealing with the need to change a burner on the fly.
While there are numerous incinerators in use currently, the inventor is not aware of any in which the system can be serviced or the burners replaced without the facility owner having to shut down operations and suffering economic losses associated therewith.
In another aspect of flare design, the height of stacks generally are often dictated by the results of environmental plume calculations. Conventional flares with external mix result in low flow discharge and must have high stacks to provided sufficient exhaust dispersion. Ground flares and incinerators are typically much shorter than conventional flare stacks and are subject to these plume or dispersion controls. Despite combustion occurring within the burn chamber of a ground flare, regulatory controls can require a ground flare to have a much greater height than is necessary only to satisfy the combustion requirement. Increased flare height results in an economic impact including the amount of material used and stack support.
Increased flow discharge from the flare positively affects the stack height requirements; the higher the discharge velocity or flow rate for a given stack size, the lower the stack height.
It is known, in the defence industry, to introduce cooling air to a stack through annular openings on exhaust stacks of ships-of-war for reducing their heat signature and thereby avoiding detection by heat-seeking missiles. The exhaust stacks were constructed of ever increasing diameter tubular shells which permitted additional ambient temperature air to co-flow with the hot exhaust, thereby cooling the exhaust stack. The ship's exhaust was fully combusted at that point and the incoming air aided only in the cooling of the stack.
In light of the above, it is a desirable characteristic to simplify the apparatus of ground flare stacks, improve combustion and to provide a highly dispersed exhaust from the flare stack without interfering with the operation of the burners.